1. Field of the Invention
Gas turbines for maximum stress. The critical component is the blade, whereby the protective coatings against erosion, wear, corrosion and oxidation at high temperatures gain in significance. Usually the protective coating has a shorter life span than the core metal of the blade; therefore, the replacement of the former moves more and more into the foreground.
The invention relates to the further development of a process for repairing, maintaining, and replacing components of thermal machines that have protective coatings and have become useless due to erosion, wear, corrosion, oxidation or mechanical damage. In this process the old protective coating has to be removed, which in principle can be achieved by a mechanical or chemical route.
As the reverse process to electroplating, the electrochemical method assumes a special position.
In particular, the invention relates to a process for electrolytically detaching a protective coating having a high Cr and Ni and/or Co content from a base metal of a component that is made of a nickel or cobalt-based superalloy.
2. Discussion of the Background
The removal of protective coatings on substrates that are made of a superalloy is performed, among other methods, by a chemical route. To date electrolytical detachment has not been appreciably used for such alloys. Some methods are known that detach the metals from their substrates by reversing the method of electroplating. It is known from US-A-2 907 700 to remove electrolytically coatings of metals (Ag, Ni, Cd, Zn, In) from a plutonium substrate. Sulfuric acid or a sodium phosphate solution is used as the electrolyte. An electrolytic process is known from DE-B 21 46 828 for detaching metal coatings (Cr, Au, Cd, Cu, Ag, Zn, Sn, Ni) from stainless Cr/Ni steel. Bromine-containing solutions of nitrates, acetates, chlorides etc. are used as the electrolyte. Apparently the attack on the substrate is negligible, According to DE-C-25 27 152 coatings of metals (Ni, Cr, Zn, Sn, Cu, Cd, Ag) are to be removed electrolytically from steel by using nitric acid or nitrate-containing solutions that contain iodine as the electrolyte, to which additional organic chlorine compounds are added.
These known methods, which are based on the fact that the detaching potential of the metal coating is adequately different from that of the substrate, cannot in the current form be transferred to protective coatings on nickel-based superalloys. The close relationship of the chemical composition between the protective coating and the substrate usually makes it impossible to electrolytically detach the latter without simultaneously attacking the substrate in an inadmissible manner. Not even falling back upon complex-forming additions to the electrolyte provides a remedial measure.
Furthermore, in the case of components that are made of a superalloy (gas turbine blade) the conditions for non-aggression with respect to the substrate is much more rigorous than for any other, e.g. aforementioned, objects. Only in a few cases could a gas turbine blade whose core metal is modified even only slightly be reused.
Therefore, there is a strong need to eliminate largely the aforementioned deficiencies and to specify means for the successful application of an electrolytic process to detach protective coatings applied on nickel-based or cobalt based superalloys.